Frequency shift keying (or digital frequency modulation) is commonly employed in data communication systems for carrying out the transmission and recovery of digital data. At the receiver the identification of the transmitted digital symbols may be accomplished coherently or by use of a non-coherent recovery system. Coherent detection employs a fixed frequency reference and a local carrier recovery loop and becomes severely degraded when the signals are subject to secondary frequency-shifting and noise. Non-coherent detection does not require a stable frequency reference but still must provide an accurate carrier tracking loop in order to follow frequency excursions between adjacent symbols. This is particularly true in frequency hopping systems where accurate carrier tracking and noise/jitter performance in the presence of large dynamic variations are required.
Conventional approaches for carrier synchronization typically derive an estimate of frequency error from the magnitudes of the spectral components derived from an MFSK detector, especially those components that are adjacent to the nominal signal frequency. Now, while these techniques offer the advantage of using the same detector for frequency error estimation as they do for data detection, because of the non-linear characteristics of these types of estimators, they are not suited for accurate tracking of large dynamics. This shortcoming is illustrated in FIG. 1 wherein curve A shows the non-linear frequency estimation characteristics of the most common Fast Fourier Transfer (FFT) based discriminator, where E is the input error (relative to the spectral spacing) and F is the frequency estimate. Similarly, the closed loop step response of a non-linear FFT estimator is depicted as curve C in FIG. 2. As shown therein, the conventional response undergoes a large dynamic swing over the entirety of the range, so that it cannot realistically track large frequency changes from symbol to symbol.